1. Field of the Invention
The present invention relates to a hydraulic controller for working machine.
2. Description of the Related Art
In hydraulic excavator, bleed-off control is used in order to put a part (surplus part) of the pressure oil discharged from a pump back to a tank.
A general bleed-off control provides a bleed-off path in the control valve provided for each of a plurality of actuators, and to change the opening area of the path according to operation amount of an operating means, which requires the control valve to be longer in a direction of a spool axis of the valve.
To overcome the foregoing deficiency, there is proposed a technique of arranging a common bleed-off valve for a plurality of control valves. One known electronic control system is to use a hydraulic pilot valve for the bleed-off valve, and to control the bleed-off valve using the secondary pressure of a proportional solenoid valve controlled by a controller (refer to Japanese Patent Laid-Open Publication No. 11-303809 for example).
In accordance with the system above, however, in the case of a failure of the proportional solenoid valve itself or an abnormality of the control system such as a disconnection of the signal system for transmitting control signal from the controller to the proportional solenoid valve, the bleed-off valve is turned to an unload position (maximum opening position) thereof to unload a total amount of oil discharged from the pump, which results in a complete halting of the machine.
Consequently, the foregoing system suffers from a problem in that the working machine could be brought to a standstill on work site.